1. Field of the Invention
The present invention relates to image-forming apparatuses that form images on sheets.
2. Description of the Related Art
Image-forming apparatuses such as copiers, printers, and facsimiles can transfer a toner image on an image-bearing member. In such an image-forming apparatus, a sheet is conveyed to a transfer portion and the toner image is transferred on the sheet.
At this time, when the toner image is transferred on the sheet at the transfer portion, a transfer condition depends on a behavior of the sheet to be transferred at the transfer portion. To stabilize the behavior of the sheet to be conveyed, various suggestions are made.
For example, a pre-transfer guide is provided at the upstream side of the transfer portion. The pre-transfer guide allows the sheet to enter the transfer portion while regulating the behavior of the sheet. Accordingly, the toner image is transferred on the sheet in a stable manner.
However, in the case where such a pre-transfer guide is provided, if the sheet is thick and hard, the posture of the sheet may be rapidly changed toward the image-bearing member when the rear end of the sheet passes through the pre-transfer guide. In such a case, a horizontal line may appear in an image to be transferred on the sheet due to a shock caused by the change in posture of the sheet.
For example, a configuration is provided in which an elastic member is provided at a tip end of the pre-transfer guide, and the elastic member reduces the rapid change in posture of the sheet so as to prevent the horizontal line from appearing on the sheet (e.g., see Japanese Patent Laid-Open Nos. 08-076607 (corresponding to U.S. Pat. No. 5,594,539) and 05-257395).
Meanwhile, when the sheet is being transferred and is in a posture such that a portion of the sheet at the upstream side of the transfer portion extends along the image-bearing member, the sheet may contact the toner image on the image-bearing member before the transfer, and accordingly, the sheet may rub the image and deteriorate the image.
The rubbing of the sheet against the toner image on the image-bearing member at the upstream side of the transfer portion may occur particularly in a configuration in which a belt serves as the image-bearing member, and a nip between the belt and a transfer roller serves as the transfer portion. To realize good transcription for various types of sheets, the contact pressure of the transfer roller with respect to the belt bearing the toner image is necessary to be increased. If the contact pressure at the transfer portion is increased, a nip width of the belt and the transfer roller at the transfer portion may be increased. Owing to this, the sheet may be conveyed to extend along an extension of the nip, or along the belt at the upstream side of the transfer portion.
In such a configuration where the belt serves as the image-bearing member, since the sheet is in the posture extending along the belt, the sheet may contact the toner image on the belt at the upstream side of the transfer portion. Owing to this, the sheet may rub the image on the belt and deteriorate the image. Even when a cylindrical photosensitive drum is used as the image-bearing member, for instance, when an image is formed on a curved sheet, the image rubbing may still occur.
To regulate the posture of the sheet, if a guide for guiding a surface of the sheet facing the image-bearing member at the upstream side of the transfer portion is arranged at a position distant from the image-bearing member, the sheet does not contact toner images on the image-bearing member before the transfer, thereby preventing the sheet from rubbing the images on the image-bearing member so that the images would not be deteriorated. However, since the position of the guide is distant from the image-bearing member, the change in posture of the sheet when the rear end of the sheet passes through the guide becomes further rapid, and the accompanying shock may increase the occurrence of a horizontal line in the images.
As described above, it is difficult to solve both problems such as the image rubbing due to the contact between the sheet and the toner images on the image-bearing member occurring while the pre-transfer guide guides the sheet from its tip end portion to its rear end portion, and the occurrence of the horizontal line due to the shock applied when the rear end of the sheet passes through the pre-transfer guide.